Hydraulic system

ABSTRACT

A hydraulic system has an adjustable hydraulic pump which feeds a hydraulic motor through a first conduit. A pressure sensitive valve is connected to the hydraulic discharge point of the motor. The pressure sensitive valve includes a spring biased closure means which communicates the pressure in the first conduit to a second conduit the latter of which cooperates with the hydraulic motor to adjust the pumping of the same. A hydraulic correcting valve is connected in series with the second conduit and has a proportional-derivative characteristic so that the motor output is initially reduced to a greater extent than thereafter in response to opening of said closure means.

United States Patent [111 3,769,800 Winter [451 Nov. 6, 1973 [54] HYDRAULIC SYSTEM 2,255,783 9 1941 Kendrick 60/450 x Inventor: Sigbert Winter Heidenheim, 3,444,689 5/1969 Budzich 60/445 X Y Primary Examiner-Edgar W. Geoghegan [7 3] Assignee: Robert Bosch GmbH, Stuttgart, Attorney-Michael S. Striker Germany [22] Filed: May 11, 1972 ABSTRACT A hydraulic system has an adjustable hydraulic pump A l. N 241 1 PP o which feeds a hydraulic motor through a first conduit. A pressure sensitive valve is connected to the hydraulic [30] Foreign Application Priority Data discharge point of the motor. The pressure sensitive May 12, 1971 Germany P 21 23 404.8 Valve n e a spring biased closure means which communicates the pressure in the first conduit to a sec- [52] US. Cl 60/445, 60/450, 60/451 0nd n i h l er f which cooperates with the hy- [51] Int. Cl. Fl6h 39/46 r li m r o just the pumping of the same. A hy- [58] Field of Search 60/381, 445, 448, li orr ing v lv is connected in series with the 6 0/450, 451 second conduit and has a proportional-derivative char- 1 act'eristic so that the motor output is initially reduced [56] References Cit d to a greater extent than thereafter in response to open- UNITED STATES PATENTS ing of d closure 2,238,061 4/1941 Kendrick 60/450 X 8 Claims, 3 Drawing Figures Patented NO). 6, 1973 2 Sheets-Sheet 1 Fig. 7

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j 1 HYDRAULIC SYSTEM BACKGROUND OF THE INVENTION The present invention relates to hydraulic systems, i

and particularly to a system having a hydraulic adjustable pump and a hydraulic motor driven by said pump and a hydraulic feedback system for adjusting the pumping action of the hydraulicpump in response to theload imposed on the hydraulic motor.

Such a hydraulic system can be utilized to regulate the flow of hydraulic fluid from a hydraulic pump to a hydraulic motor to regulate the same at a substantially constant speed or to a hydraulic cylinder to operate the same at a substantially constant thrusting speed. In each case, the speed of rotation of the hydromotor or the thrusting speed of the hydraulic cylinder,the actual value of the operating parameter is, in the first instance, controlled by the amount .of fluid which the pump conveys.

In hydraulic systems of this type, it often happens that during regulation of the pumping action of the hydraulic pump in response to a deviation from a desired value of an operating parameter, an adjusting or positioning device which controls the pumping action of the pump operates thereon for a period of time'which is longer than necessary. Such results frequently occur because of the time constants involved with such control systems. The result of such prolonged action is that the desired value of the parameter is often exceeded or surpassed. Accordingly such regulation is not particularly. accurate and is often susceptible to oscillations about the desired value. For these reasons, such hydraulic systems are not suitable for all purposes.

SUMMARY OF THE INVENTION -A'ccordingly, it isan object of the present invention to provide a hydraulic system which does not have the disadvantages known in systems of this'type- It is another object of the present invention to provide a hydraulic system which is simple in construction and economical to manufacture and which can accurately control the operation of the hydraulic pump.

It is still another object of the present invention to provide a hydraulic system of the type under discussion which can regulate the pumping action of an adjustable hydraulic motor as-a function of both the rate of flow of the hydraulic fluid so pumped or the hydraulic pressures which are developed in said fluid.

It is a further object of the present invention to provide a hydraulic system wherein a hydraulic pump feeding a hydraulic motor can easily and accurately be adjusted as a function of the load placed on the hydraulic motor. It is still a further object of the presentinvention to provide a hydraulic system which accomplishes the above objects by regulation action of the hydraulic pump through proportional-derivative action based on the parameter of interest.

With the view of achieving the above objects, the

. present invention for a hydraulic system comprises, in

. j 2 amount of hydraulic fluid delivered by said adjustable hydraulic pump including second conduit means connecting said hydraulically actuated adjusting means with said first conduit means sothat said hydraulically actuated adjusting means regulate said pump in dependence upon fluid pressure in said first conduit means. Closure means are provided in said second conduit means and means for opening said! closure means in dependence upon the fluid pressure in said first conduit means after said hydraulic motor are provided. Hydraulic. correcting means are arranged in said conduit means and constructed so that the hydraulic fluid passing through said second conduit means from said first conduit means to said adjusting means will exert, during-a short period after opening said closure means, a higher regulating pressure on said hydraulically actuated adjusting means than thereafter, so that during said period of air pressure the output of said adjustable pump is reduced to a greater extent than thereafter.

According to a presently preferred embodiment, said hydraulic correcting means comprises a hydraulic proportional-derivative controller which comprises combination, an adjustable hydraulic pump adapted to first and second parallel fluid paths. The paths are com nected in series with said second conduit means. Said first fluid path includes first pressure responsive means arranged to transmit substantially instantaneous pressure changes from one end to the other end of said second conduit means. Said second fluid path includes second pressure responsive means arranged to transmit pressures through one end of said second conduit means which are proportional to the pressures at the other end thereof. More particularly, said first pressure responsive means comprises a piston having two pressure bearing surfaces slidable in said first fluid path between two end positions in response to pressure differentials on said surfaces of said piston. Biasing means for positioning said piston in a normal predetermined position are provided for restoring said piston to said predetermined position when the pressures have equalized in said second conduitmeans at said'two surfaces. Said second pressure responsive means comprises a'pressure regulating valve which is adjustable.

According to another featureof the invention, biasing means are provided which cooperate with said closure means to maintain the latterrclosed, said biasing means being adjustable, whereby the opening of said closure means is a function of both the pressure in said first conduit means and the adjustment of said biasing means.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. 3 is a side elevational view, in cross-section, of

j a pressure sensitive valve utilized in the hydraulic system shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, wherein similar reference numerals have been used to designate similar parts throughout, and first referring to FIG. 1, the reference numeral 1 designates an adjustable hydraulic pump having a positioning element 2. The positioning element 2 controls the amount of hydraulic fluid pumped by the hydraulic pump 1 and may be provided with biasing means (not shown) which returns the positioning element 2 to a normal position wherein the adjustable hydraulic pump 1 pumps its rated or maximum amount of fluid needed per unit time.

Connected to-the positioning element 2 is a hydraulically actuated adjusting means 3 whose output is connected to the positioning element 2 and which is movable to adjust the positions of the latter in response to changes in hydraulic pressure at its input.

A primary conduit 4 connects the hydraulic pump 1 with a hydraulic motor 5. The hydraulic motor 5 is operated by'the flow of hydraulic fluid passing through the primary conduit 4 under pressure established by the hydraulic pump 1. After passing through the hydraulic motor 5, the hydraulic fluid passes through a pressure sensitive valve 8,'shown schematically and to be described more fully with respect to FIG. 3, to be discharged through an outlet conduit 6' into a hydraulic fluid reservoir 7.

The pressure sensitive valve 8 further includes a channel means 24, to be further described in connection with FIG. 3, which communicates the primary conduit 4 with a secondary conduit 9. The secondary conduit 9 forms part of a feedback path of the hydraulic system and is series-connected with a hydraulic correcting valve 10 which forms one of the important features of the present invention. The hydraulic correcting valve 10, as will become evident from the description below, imparts a proportion-derivative characteristic to the operation of the hydraulic system, andparticula'rly the feedback path thereof to thereby stabilize the system in a desirable manner.

The hydraulic correcting means 10 generally comprises a housing 10' which is provided with two parallel fluid paths 11 and 12. Connected in series with the fluid path 11 is a pressure regulating valve 11a which is adjustable in a conventional manner.

The fluid path 12 is provided with shoulders 12a and 12b at its ends. A piston 13 is slidably mounted within the fluid path 12 and is configurated to seal the two portions of the fluid path 12 whichit forms. The piston 13 has pressure bearing surfaces 13a and 13b which are exposed to the hydraulic fluid present at each side of the piston 13. The hydraulic fluid applies pressures to the bearing surfaces 13a and 13b to thereby produce a differential pressure on the piston 13 which causes it to slide in one direction or another.

Biasing means in the form of springs 14 and 15 are respectively provided in a somewhat compressed state between the shoulder 12a and the pressure bearing surface 13a, on the one hand, and between the pressure bearing surface 13b and the shoulder 12b, on the other 15 are so selected that with equal pressures against the bearing surfaces 13a and 13b of the piston 13, the same is positioned in a normally median position in the fluid path 12. When a differential pressure acts upon the piston 13, the piston moves in a direction corresponding to the direction of the differential pressure, said springs 14 and 15 restoring the normally median position of the piston 13 as soon as the differential pressure is removed. In accordance with the presently preferred embodiment, the springs 14 and lime identical to each other and are equally compressed to thereby provide a symmetrical biasing arrangement about the piston 13.

Channels 16 and 17 connect to the respective ends of the fluid paths -l1 and 12 to thereby bring these fluid paths into fluid communication with each other. The channel 17 is in fluid communication with an inlet opening 18 of the hydraulic correcting valve-10. The secondary conduit 9 discharges into the inlet opening 18. On the other hand, the channel 16 is in fluid communication with the outlet opening 19 of the hydraulic correcting valve 10 and the outlet opening 19 is in fluid communication with the input of the hydraulically actuated adjusting means 3. The connection between the outlet opening 19 and the hydraulically actuated adjusting means 3 is through a conduit 20 which may be considered a part of the secondary conduit 9, and

wherein said hydraulic correcting valve 10 is connected in series with said secondary conduit.

Referring to both FIGS. 1 and 3, the pressure sensitive valve 8 is provided with a cylindrical bore which hand, respectively. The constants of the springs 14 and has a smooth internal surface. A control plunger 31 is arranged within the bore 30 for sliding movement in directions along the axis of the bore 30. The control plunger 31 forms a closure means, as will presently be described. The control plunger 31 is also provided with an axial bore 32 which has at least an upper smooth surface 31a. The lower portion 31b of the axial bore 32 is threaded as shown. These threads are meshed with a cylindrical damper restriction 23 which itself is provided with a small diameter central bore 23a. A pipe 23 having an upper threaded portion 33a is connected to the upper portion of the pressure sensitive valve 8 as shown to provide a fluid tight seal. The lower portion 33b of the pipe 33 has a smooth external surface and has an outer diameter which is substantially equal to the inner diameter of thesmooth inner surface 31a. In the same manner,'the outer diameter of the control plunger 31 ismade substantially equal to the inner diameter of the bore 30 so ,as to provide fluid tight seals between contiguous surfaces so as to prevent passage of hydraulic fluid therebetween. To further ensure the fluid tight sealed relationship between the smooth portion 33b of the pipe 33 and the inner surface 310 of the control plunger 31, an annular sealing ring 310 is provided in an annular slot of the control plunger 31 as shown and the annular sealing ring 31c is made of resilient substance or material whose inner diameter is at least equal to the outer. diameter of the smooth portion 33b but is preferably smaller than the latter. In this manner, upon insertion of the pipe 33 through the annular sealing ring 310, the latter deforms to some extent and its inner diameter becomes lodged to thereby intimately engage the smooth portion 33b of the pipe 33 to thereby ensure that no fluid can pass in whatever clearance which may exist between a smooth portion 33b and the smooth inner surface 31a. I

The pipe 33 is provided with a central bore which is coaxial with the axis of the control plunger 31. A conduit. 6, which is connected to the discharge output of the hydraulic motor 5 discharges into the pipe 33.

Provided in the region of the pipe 33, an opening 34 is provided in the pressure sensitive valve 8 which communicates with a cylindrical cavity 34a formed by the pressure sensitive valve housing, the smooth portion 33b as well as the upper surface of the controlplunger 31. Because of the fluid tight relationships described above, the cylindrical cavity 34a can only receive or discharge fluid through the bore 34. A conduit 22, in fluid communication with the conduit 6, discharges into the bore 34.

Formed along the inner surfaces of the bore 30 of the pressure sensitive valve 8 are first and second annular slots 35 and 36 which are in communication with the exterior of the housing by means of bores 37 and 38, respectively. A conduit'21, in communication with the primary conduit 4, discharges into the bore 37 while the secondary conduit 9 is in fluid communication with the bore 38. I

The control plunger 31 has a channel 24 parallel to the axis of the bore 30. Two openings 24a and 24b communicate the channel 24 to the exterior of the control plunger 31, said openings being spaced a distance equal to the, distance between bores 37 and 38. A plug 240 closes the channel 24 at the point where it was formed, e.g. by drilling.

The pressure sensitive valve housing 8 is provided with a cover 39 at one end thereof which has a bore 39a which is in fluid communication with the axial bore 32. The bore 39a is in fluid communication with the conduit 6' which discharges, as described above, into the reservoir 7.

The restriction 23 prevents sudden or abrupt responses in the movement of the control plunger 31 and therefore operates as a damping element on the control plunger 31. The amount of damping which the restriction 23 provides is a function of the diameter of the bore 23a. The restriction 23 therefore has the advantageous effect of reducing oscillatory motion of the control plunger 31 which would be undesirable in many applications.

Referring to FIGS. land 3, the spring 25 is adjustable tothereby make it possible to select a desired pressure in the primary conduit 4. Although the spring '25 has not been shown to be adjustable in FiG. 3, the present invention contemplates that the spring can be made so adjustable in any known or conventional manner.

" For the purposes of describing the present application, and for the claims which follow the detailed description, a first conduit means is to be defined as a conduit means including theconduits 26, 4, 6 and 6'. 0n the other hand, a secondfconduit means is to be defined as includingsecondary conduit 9 and conduit 20.

The operation of the present invention will now be described. In response to operation of the hydraulic faster and slower speeds about a preselected'desired.

rotating speed. Assuming that there is some load on the hydraulic motor 5, the pressure in the conduit 6 will generally be lower than that in the conduit 4. However, at no load condition of the hydromotor, the pressures in the latter two conduits approach one another.

' With the hydraulic motor operating at its desired speed, the hydraulic fluid leaving the hydraulic motor passes through the conduit 6, through the pipe 33, through the control plunger 31, the damper restriction 23 and finally through a conduit 6' to the reservoir 7. The passage of the hydraulic fluid through the damper restriction 23, causes the control plunger 31 to be urged in a downward direction. However, such downward movement of the control plunger 31 is resisted by the helical spring 25 and only downward forces above a predetermined value are effective to move the control plunger 31 sufficiently whereby the channel 24 places the conduits 21 and 9 into fluid communication with one another.

The pressure in the conduit 6 is monitored by the conduit 22 and this pressure is provided in the cylindrical cavity 34a. Although the conduit 22 has been illustrated as being connected to the conduit 6, it is within the scope of the present invention that the conduit 22 be connected directly to the hydraulic motor 5, at a suitable place thereof, wherein the hydraulic fluid has already been utilized to turn the blades of the hydraulic motor. i

Under load, the pressure in the conduit 6, drops because of the decrease in the rate of flow of hydraulic fluid through the hydraulic motor 5. Therefore, the pressure differential between the conduits 4 and 6 increases. However, because of the low pressure in the conduit 6, a simila'rly low pressure will existwithin the cylindrical cavity 34a and therefore a relatively small pressure will be. exerted against the control plunger 31 in a downward direction. Therefore, the control plunger 31 will only .move a small distance in the downward direction which is, however, not sufficient to place the conduits 21 and 9 into fluid communication. Therefore, it will be noted that with this arrangement, when the hydraulic motor is operating at or near full load, the hydraulic pump 1 is required to operate in its rated or maximum rate of fluid supply.

On the other hand, when the load on the hydraulic motor 5 is made very small or totally eliminated, then the pressure in the conduit 6 approaches that existing in the primary conduit 4 and, correspondingly, the pressure provided in the cylindrical cavity 34d likewise increases. In response to increased pressure in the cylindrical cavity 34a, the control plunger 31 will move in a downward direction and, depending on the adjustmentof the biasing forces due to the helical spring 25, the control plunger 31 will move sufficientlydownward to place the conduits 21 and 9 into fluid communication with each other and with the primary conduit 4. Referring to FIG. 3, this occurs when the channel 24 moves to a position wherein the openings 24a and. 24b

.meet the bores 37and 38. Such a meeting enables the 9 in fluid communication with each other. The application of a sudden change in fluid pressure at the inlet opening 18 as well as in the channel 17 has two effects.

First, as soon as a pressure is developed at the pressure bearing surface 13b which is larer than the pressure appearing at the pressure bearing surface 13a, the differential in pressures causessaid piston 13 to move towards the left, as viewed in FIG. 1, such movement simultaneously increasing the pressure in the channel 16 as well as the outlet opening 19. The movement of the plunger 13 is almost instantaneous, and therefore, the responsiveness of the piston 13 to abrupt changes in differential pressures applied thereto is a function of the mechanical time constants involved in the circuit. For these reasons it is preferred that the mass of the piston 13 be made as low as possible. Thus, with such an arrangement, it is noted that input step functions of the pressure in the conduit 9 will be transmitted in the form of impulses by way of the fluid path 12.

On the other hand, a step increase in pressure at the inlet opening 18 is also transmitted through the fluid path 11 and particularly the pressure regulating valve 110. While the piston 13 operates as a differentiating controller, the pressure regulating valve 11a operates as a proportional controller so that at least for the time when the channel 24 places the conduits 21 and 9 in fluid communication with each other, the higher pressure existing at the right-hand side of the pressure regulating valve 11a is proportionally communicated to the left-hand side thereof until the pressures in channels 16 and 17 have equalized. As described above, during this condition, the piston 13 has again moved to its median position shown .in FIG. 1. By placing the paths l2 and i 11 in parallel to each other and providing a different pressure responsive means in each of the channels, desirable operating and control characteristics of the hydraulic system can be obtained. More particularly, the type of control described above includes both derivative as well as proportional control. For this reason, the hydraulic correcting valve may be defined as a proportional-derivative hydraulic controller.

Now, for the small or no load condition of the hydraulic motor 5, the abrupt movement of the piston 13 causes the hydraulically actuated adjusting means to at least initially cause the adjustable hydraulic pump 1 to pump less fluid than would normally be desired. However, after the pressures in the channels 16 and 17 have equalized, the positioning element 2 comes to rest in a position corresponding to the small or no load operation which is manifested in the'pressure in conduit 4. Such subsequent increase in the pump output is urged by biasing means (not shown) which cooperate with said positioning element 2 to restore the same towards a normal, full load position.

However, it is clear from the above description that at no load or near zero load on the hydraulic motor, the present hydraulic control system will have theeffect of making the adjustable hydraulic pump I operate at less than its full load capacity.

Although the hydraulic correcting valve 10 has been described as providing a proportional-derivative control action in the feedback conduit of the hydraulic system, the present invention also contemplates the utilization of proportional-integral or proportional- ,integral-derivative hydraulic controllers where such alternate forms of controllers are more suitable to properly stabilize a system. In the case of a three action controller, it may be necessary to add a third parallel path similar to paths l1 and 12 including pressure responsive means which provides an output which is a function of the integral of the input pressure. Naturally, if

only proportional-integral action were desired, then the path 12 as well as the piston 13 and the springs 14, 15 would be deleted and the same would be replaced by an integral responsive means.

Referring to FIG. 2, this shows a schematic block diagram of the hydraulic system, showing the characteristics of the respective elements of the system abovedescribed. Here, the hydraulic pump 1 and the hydraulically actuated adjusting means 3' have been combined and are shown to have a linear response. Thus, the output of the hydraulic pump 1 is directly a function of the pressure applied to the input Q of the hydraulically actuated adjusting means 3. The hydraulic correcting means 10' is represented by a characteristic curve which illustrates that during a short time period after the opening of the closure means, which comprises the control plunger 31, a higher regulating pressure is applied to said hydraulically actuated adjusting means than thereafter, so that during said period of high pressure, the output of said adjustable pump 1 is reduced to a greater extent than thereafter. The characteristic curve of the pressure sensitive valve 8 is shown to be a step function since, ideally, the pressure Q in the secondary conduits 9 is substantially equal to zero when there is no fluid communication between the conduit 21 and 9 to the channel 24, the pressure O in the conduit 9 rising steeply in the critical position of the control plunger 31 when such fluid communication is achieved to a value which is substantially equal to the pressure existing in the primary conduit 4.

The pressure sensitive valve 8', illustrated schematically in FIG. 2, is shown to have two inputs A and B. The input A is in actuality theoutput from the hydraulic motor 5', the input of the latter being connected to the hydraulic pump, hydraulically actuated adjusting means combination 1', 3 respectively. The input B, represents a desired value at the output of the hydraulic motor 5'. Referring to FlG. 3, the actual value A represents the pressure which exists in cylindrical cavity 34a and which is derived from the conduit 6 which represents the pressure at the output of the hydraulic motor 5. The pressure which represents the actual value of the output pressure of the hydraulic motor operates on the plunger 31 to move the same in a downward direction. On the other hand, the helical spring 25, which represents the desired value B, preferably adjustable, acts on the control plunger 31 in an upward direction. For this reason, the actual value A has been designated by a plus sign and the desired value B has been designated by a negative sign, these two signs illustrating that the desired and the actual values operate in different directions or are out of phase insofar as the pressure sensitive valve 8' is concerned.

Although the present invention has been described in connection with the flow of hydraulic fluid from a pump for utilization in driving a hydraulic motor, it is also contemplated by the present invention that not only the quantity of hydraulic fluid pumped by a hydraulic pump can be regulated in this manner, but also the pressure at the output of a hydraulic pump can similarly be regulated. Thus, for example, instead of hydraulic fluid valves, pressure valves may be utilized instead.

With the above-described features of the present invention, as well as with others which will become readily apparent to those skilled in the art, it is possible to stabilize hydraulic circuits which are different to otherwise control in a satisfactory manner with very simple means. More importantly, the objects of the present invention can be achieved by the utilization of a hydraulic correcting valve as described above which is placed in the feedback path of the hydraulic system and which exhibits a proportional-derivative action in response to a change in pressure in the system to thereby regulate first abruptly, and then more gradually, the pumping action of an adjustable hydraulic pump.

it will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of hydraulic systems differing from the types described above.

While the invention has been illustrated and described as embodied in a hydraulic system for the regulation of a hydraulic pump which drives a hydraulically driven component, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended l. A hydraulic system comprising, in combination, an adjustable hydraulic pump adapted to deliver hydraulic fluid; first conduit means connected to said adjustable hydraulic pump; a hydraulic motor arranged in said first conduit means and driven by said hydraulic fluid delivered by said adjustable hydraulic pump; hydraulically actuated adjusting means for reducing the amount of hydraulic fluid delivered by said adjustable hydraulic pump including second conduit means connecting said hydraulically actuated adjusting means with saidfirst conduit means so that said hydraulically actuated adjusting means regulates said pump in dependence upon the fluid pressure in said first conduit means; closure means in said second conduit means; means for opening said closure means in dependance upon the fluid pressure in said first conduit means after said hydraulic motor; and hydraulic correcting means arranged in said second conduit means and constructed so that the hydraulic fluid passing through said second conduit means from said first conduit means to said adjusting means will exert, during a short time period after opening of said closure means, a higher regulating pressure on said hydraulically actuated adjusting means than thereafter, so that during said period of higher pressure the output of said adjustable pump is reduced to a greater extent than thereafter.

2. A system as defined in claim 1, wherein said hydraulic correcting means comprises a hydraulic proportional-derivative controller.

3. A system as defined in claim 1, wherein said hydraulic connecting means comprises first and second parallel fluid paths connected in series with said second conduit means, said first fluid path including pressure responsive means arranged to transmit substantially instantaneous pressure changes from one end to the other end of said second conduit means, and said second fluid path includ-ing pressure regulating means arranged to transmit pressures to one end of said second conduit means which are proportional to the pressures at the other end thereof.

4. A system as defined in claim 3, wherein said pressure responsive means comprises a piston having two pressure bearing surfaces slidable in said first fluid path between two end positions in response to pressure differentials on said surfaces of said piston; and biasing means for positioning said piston in a normal predetermined position and for restoring said piston to said predetermined position when the pressures have equalized in said second conduit means at said two surfaces.

5. A system as defined in claim 4, wherein said first fluid path includes two shoulder portions in the regions of its ends, and wherein said biasing means comprises a pair of helical springs, each spring being compressively positioned between a respective shoulder portion and a pressure bearing surface of said piston.

6. A system as defined in claim 3, wherein said pressure regulating means comprises :a pressure regulating valve.

7. A system as defined in claim 6, wherein said pressure regulating valve is adjustable.

8. A system as defined in claim 1, further comprising biasing means cooperating with said closure means to maintain the latter closed, said biasing means being adjustable, whereby the opening of said closure means is a function of both the pressure in said first conduit means and the adjustment of said biasing means. 

1. A hydraulic system comprising, in combination, an adjustable hydraulic pump adapted to deliver hydraulic fluid; first conduit means connected to said adjustable hydraulic pump; a hydraulic motor arranged in said first conduit means and driven by said hydraulic fluid delivered by said adjustable hydraulic pump; hydraulically actuated adjusting means for reducing the amount of hydraulic fluid delivered by said adjustable hydraulic pump including second conduit means connecting said hydraulically actuated adjusting means with said first conduit means so that said hydraulically actuated adjusting means regulates said pump in dependence upon the fluid pressure in said first conduit means; closure means in said second conduit means; means for opening said closure means in dependance upon the fluid pressure in said first conduit means after said hydraulic motor; and hydraulic correcting means arranged in said second conduit means and constructed so that the hydraulic fluid passing through said second conduit means from said first conduit means to said adjusting means will exert, during a short time period after opening of said closure means, a higher regulating pressure on said hydraulically actuated adjusting means tHan thereafter, so that during said period of higher pressure the output of said adjustable pump is reduced to a greater extent than thereafter.
 2. A system as defined in claim 1, wherein said hydraulic correcting means comprises a hydraulic proportional-derivative controller.
 3. A system as defined in claim 1, wherein said hydraulic connecting means comprises first and second parallel fluid paths connected in series with said second conduit means, said first fluid path including pressure responsive means arranged to transmit substantially instantaneous pressure changes from one end to the other end of said second conduit means, and said second fluid path includ-ing pressure regulating means arranged to transmit pressures to one end of said second conduit means which are proportional to the pressures at the other end thereof.
 4. A system as defined in claim 3, wherein said pressure responsive means comprises a piston having two pressure bearing surfaces slidable in said first fluid path between two end positions in response to pressure differentials on said surfaces of said piston; and biasing means for positioning said piston in a normal predetermined position and for restoring said piston to said predetermined position when the pressures have equalized in said second conduit means at said two surfaces.
 5. A system as defined in claim 4, wherein said first fluid path includes two shoulder portions in the regions of its ends, and wherein said biasing means comprises a pair of helical springs, each spring being compressively positioned between a respective shoulder portion and a pressure bearing surface of said piston.
 6. A system as defined in claim 3, wherein said pressure regulating means comprises a pressure regulating valve.
 7. A system as defined in claim 6, wherein said pressure regulating valve is adjustable.
 8. A system as defined in claim 1, further comprising biasing means cooperating with said closure means to maintain the latter closed, said biasing means being adjustable, whereby the opening of said closure means is a function of both the pressure in said first conduit means and the adjustment of said biasing means. 